The surface tension of the aqueous solution of sodium 4'-(butylamino)azobenzene-4-sulfonate (SBAS) and octyl methyl sulfoxide (OMS) mixture was measured as a function of the total molality at various compositions and at 298.15 K under atmospheric pressure. The experimental results were analyzed by applying the thermodynamic relations. The phase diagrams of adsorption and micelle formation were found to suggest that SBAS and OMS molecules interact attractively with each other to form a negative azeotropic mixture in the adsorbed film at a low surface tension and in the micelle. These findings were qualitatively similar to those of the system of SBAS and 2-(octylsulfinyl)ethanol (OSE) and confirmed our conclusion that the strong interaction of dye and nonionic surfactant molecules at the interface may be the one of reasons why a nonionic surfactant acts as the leveling agent in dyeing process.
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